Thermal expansion
The pressure increase that occurs when heated water expands inside a plumbing system that has been closed off from the supply by a backflow preventer or check valve.
Thermal expansion is the increase in water volume that occurs as water is heated. In an open system the extra volume simply pushes back toward the supply, but installing a backflow preventer, check valve, or pressure reducing valve with an internal check creates a closed system. Once the system is closed, water heated by a water heater or boiler has nowhere to go, and the trapped expansion translates directly into rising pressure inside the building piping.
This matters because backflow protection and thermal expansion problems arrive together. A building that never had pressure complaints can develop them immediately after a containment assembly is installed at the meter. Symptoms include the water heater's temperature and pressure relief valve weeping, faucets briefly spitting at high pressure, running toilets, and stressed supply connectors. The backflow preventer is doing its job; the system simply was never designed to be closed.
Testers and property owners should treat thermal expansion control as a standard companion to backflow protection. The usual remedy is a properly sized expansion tank or another approved expansion control device on the heated water system, giving the extra volume a cushion to move into. When installing or inspecting a containment assembly, it is good practice to ask whether expansion control exists downstream, and to alert the owner if the heater's relief valve shows signs of regular discharge.
Related terms
| Backpressure | Backflow caused when pressure downstream of the potable connection becomes greater than the pressure of the supply itself. |
| Check valve | A valve that allows water to flow in one direction and closes to prevent flow in the reverse direction. |
| Containment vs. isolation | Two complementary protection strategies: containment protects the public main at the service connection, while isolation protects users inside the building at each hazard. |
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